A single-composition emission-tunable phosphor BaY₂Si₃O₁₀:0.01Bi³⁺,mEu³⁺ has been prepared by the conventional high temperature solid-state reaction. The X-ray diffraction (XRD), luminescent properties and lifetimes are used to characterize the obtained phosphor samples. Upon near UV light excitation, BaY₂Si₃O₁₀:0.01Bi³⁺ and BaY₂Si₃O₁₀:0.13Eu³⁺ phosphors show the typical emissions of Bi³⁺ (³P₁ → ¹S₀, blue and green) and Eu³⁺ (⁵D₀ → ⁷F₂, red), respectively. The spectral analysis indicates that there exist two different Bi³⁺ luminescent centers (Bi(I) and Bi(II)) due to Bi³⁺ ions occupying different crystallographic sites in the BaY₂Si₃O₁₀ host lattice, and the energy transfer between two types of Bi³⁺ ion luminescent centers was observed. The significant spectral overlap between the emission spectrum of Bi³⁺ ions and the excitation spectrum of Eu³⁺ ions implies the occurrence of energy transfer from Bi³⁺ to Eu³⁺ and the efficient energy transfer from Bi³⁺ to Eu³⁺ has been investigated in detail. The mechanism of energy transfer from Bi³⁺ to Eu³⁺ has been discussed by using Dexter's theory, demonstrating to be a resonance type via dipole–quadrupole interaction. Moreover, the critical distance for energy transfer from Bi³⁺ to Eu³⁺ has been calculated to be 14.38 Å using the concentration quenching method. The white light emission of BaY₂Si₃O₁₀:0.01Bi³⁺,mEu³⁺ phosphors can be realized by systematically adjusting the activator dopant concentrations and utilizing the energy transfer under a near UV light pump. The quantum efficiency of BaY₂Si₃O₁₀:0.01Bi³⁺,mEu³⁺ phosphors is measured, and the samples exhibit excellent quantum efficiency. The Commission International de I'Eclairage (CIE) chromaticity coordinates, correlated color temperature (CCT) and temperature dependence emissions are also discussed in detail. The energy transfer efficiency between Bi(I) and Bi(II) changes with increasing temperature due to a phonon-assisted process. All the results indicate that the white light emitting single-composition phosphor BaY₂Si₃O₁₀:0.01Bi³⁺,mEu³⁺ has potential applications in white light LEDs.